1. Field of the Invention
The present invention relates to an improvement of a radio-frequency identification tag (RFID-tag) communication device capable of radio communication with radio-frequency identification (RFID) tags for writing and reading information on and from the RFID tags.
2. Description of the Related Art
There is known an RFID system (radio-frequency identification system) wherein an RFID-tag (radio-frequency identification tag) communication device (e.g., an interrogator) reads out information, in a non-contact fashion, from small RFID tags (radio-frequency identification tags, e.g., transponders) on which information is written. In this RFID system, the RFID-tag communication device is capable of reading out the information from the RFID tags, even where the RFID tags are contaminated or located at positions invisible from the radio-frequency tag communication device. For this reason, the RFID system is expected to be used in various fields, such as management and inspection of articles of commodity.
Generally, the RFID-tag communication device is arranged to effect radio communication with the RFID tags, by transmitting a suitable transmission signal from its antenna toward the RFID tags, and receiving through the antenna a reply signal which is transmitted from the RFID tags in response to the signal received from the RFID-tag communication device. A leakage signal that is a part of the transmission signal which is transmitted from the RFID-tag communication device toward the RFID tags and returned to and received by the communication device and which has a relatively high intensity may be mixed with the reply signal transmitted from the RFID tags, so that the intensity of the signals received by the RFID-tag communication device may exceed a permissible upper limit of an amplifier provided in the RFID-tag communication device, whereby the received signals cannot be sufficiently amplified by the amplifier, and the reply signal component cannot be sufficiently amplified. Thus, the known RFID-tag communication device suffers from a risk of insufficiency of the signal-to-noise ratio. In view of this problem, there have been proposed techniques for suppressing the leakage signal. JP-8-122429 A discloses an example of an interference compensating device used in a mobile identification system.
However, the conventional technique to suppress the leakage signal due to the transmission signal transmitted from the RFID-tag communication device and returned to the communication device uses primarily analog processing to control the amplitude and phase of a compensating signal (canceling signal) to suppress the leakage signal, and therefore requires a comparatively expensive large phase shifter. The conventional technique has a further problem of difficulty of control of the phase shifter. Thus, there has not been a simple RFID-tag communication device developed for suppression of the leakage signal.